The present invention relates to an electrolyte/electrode assembly used in a solid electrolyte fuel cell, and more particularly, to a so-called substrate structure type of electrolyte/electrode assembly in which a thin, flat solid electrolyte element and thin, flat electrodes are applied to a thick porous substrate.
A solid electrolyte fuel cell is a fuel cell which uses a solid electrolyte element, such as solid zirconia, to operate the fuel cell at high temperatures, for example, temperatures ranging between about 800.degree. and about 1000.degree. C. Compared to other known types of fuel cells, the solid electrolyte fuel cell has no problems caused by retention and corrosion of the electrolyte and eliminates the need for a catalyst to reduce any activating overvoltage during operation.
Two types of planar fuel cells have been developed. One is a conventional thin tri-layer (anode/electrolyte/cathode) design which is called a self-supported structure. The other is a new design which is called a substrate structure, in which, a thin, flat, solid electrolyte element and a thin, flat cathode are applied to a thick, porous anode substrate. The self-supported structure has a higher power density than the substrate structure because the thick substrate of the substrate structure hinders gas diffusion, which restricts current density. The thin electrode of the self-supported structure does not hinder gas diffusion. Its thin structure, however, is mechanically too weak to permit large cell areas, and the maximum electrolyte plate size is limited to about 20.times.20 cm with a thickness of 0.2 to 0.3 mm. Therefore, the self-supported structure is restricted to military or space applications where small, compact, high power density is required.
On the other hand, the substrate structure permits the fabrication of a large electrolyte plate because the thick anode can support the large but thin electrolyte plate. Electrolyte plates as large as 40.times.40 cm can be obtained with 2 to 3 mm thick anodes, although the power density will be lower than that of similar self-supported structures for the reasons given above. Therefore, a large electrolyte plate makes it possible to construct large capacity fuel cells, which allows the substrate structure to be extended to application in power plants for dispersed or central power stations. Some problems nevertheless remain to be solved in this regard.
FIG. 1 is an exploded view of a conventional electrolyte/electrode assembly of a substrate structure type fuel cell. In this assembly, a thin, flat electrolyte 2 and thin, flat cathode 3 are applied to a thick anode substrate 1. An alternate design using a thick porous cathode as a cathode substrate is available if the porous cathode is mechanically strong. Anode substrate 1, serving also as an anode, is a porous substrate with ribs and consisting of an electrolyte such as zirconia. An electrically conductive material consisting of nickel or nickel-zirconia cermet is supported in the porous substrate in order to provide electrical conductivity to anode substrate 1 in the direction of the thickness of the anode substrate. Cathode 3 consists of lanthanum manganite, LaMnO.sub.3. Usually yttria-stabilized zirconia, YSZ, is used for solid electrolyte element 2.
The electrolyte/electrode assembly thus obtained is assembled with a ribbed interconnection shown FIG. 1 to construct a unit cell. Interconnection is formed by a cathode substrate 4 consisting of LaMnO.sub.3 and a separator 5 consisting of La(Ca)CrO.sub.3. Separator 5 is formed in a layer doped with calcium on cathode substrate 4. A fuel cell stack is fabricated in a known manner by alternating the above mentioned electrolyte/electrode assemblies and the separator in sequence and by attaching fuel and air manifolds to the sides of the stack.
Conventionally, anode substrate 1 and cathode substrate 4 are made of nickel-zirconia oxide, NiO-YSZ, powder, and lanthanum manganite, LaMnO.sub.3, powder, as the raw materials for the respective parts. There parts are formed by pelletizing molding, sheet molding, extrusion molding or cold isostatic pressing (CIP), and by sintering under an oxidizing or reducing atmosphere. Usually, the NiO-YSZ anode substrate is reduced inside a fuel cell by flowing a fuel gas during the operation.
The NiO-YSZ anode substrate undergoes a five to six percent volume contraction due to the reduction. This contraction is observed even when the Ni content is lowered to approximately 30% by volume at which point electrical conductivity can be assured during the reduction. A problem that arises is that when a dense YSZ solid electrolyte layer is formed on this NiO-YSZ anode substrate, the YSZ layer develops cracks because of the changes in NiO volume during the reduction, which eventually result in warpage and cracking in the electrode substrate itself. Furthermore, in the case of an NiO-YSZ anode substrate with a diameter of at least 100 mm, the substrate itself also develops cracks by simply forming the YSZ layer. This indicates that the porous anode substrate is incapable of absorbing the difference between the coefficient of thermal expansion in NiO-YSZ of (12-14).times.10.sup.-6 .degree.C. (30.degree.-1000.degree. C., in air) and the coefficient of thermal expansion in the solid electrolyte element, YSZ of 10.5.times.10.sup.-6 /.degree.C.
The above mentioned contraction is also observed when the cathode is utilized as the substract for the electrolyte. The LaMnO.sub.3 cathode substrate also develops large volume contractions. When a dense YSZ layer is formed on this LaMnO.sub.3 cathode substrate, the YSZ solid electrolyte layer develops cracks because of the changes in LaMnO.sub.3 volume, which eventually result in warpage and cracking in the electrode substrate itself. This indicates that the porous cathode substrate is incapable of absorbing the difference between the coefficient of thermal expansion in LaMnO.sub.3 of 12.times.10.sup.-6 /.degree.C. (30.degree.-1000.degree. C., in air) and the coefficient of thermal expansion in the solid electrolyte element, YSZ, of 10.5.times.10.sup.-6 /.degree.C.